Almost all pneumatic tires are given a speed rating based on the maximum speed capability of the tire. The speed rating in conventional use today was developed in response to the need to control the safe performance of tires at standardized speeds. When a vehicle manufacturer specifies tires, the required speed rating for the tire is dictated by the type of vehicle. For a family type sedan car, the speed rating of a tire will likely be lower than the speed rating for a high performance sports car. Current speed ratings begin at 50 km/h, a B speed rating, and go to a Y or ZR speed rating for tires capable of 300 km/h and above. The majority of passenger tires have a speed rating of either SR, 180 km/h, or HR, 210 km/h.
With an ever increasing interest in high performance sports cars, and the desire to drive faster, as permitted on parts of the Autobahn in Germany, the goal is to increase the speed performance of the tire. However, as the tire rotates at a faster speed, the centrifugal force experienced by the tire and the tire components significantly increases. Designing a tire to compensate and withstand these subjected forces can involve modifying many factors, including the belt structure, the mold structure, and even the footprint shape. As the speed rating increases, small but incremental changes can yield significant increases in the tire performance and increase the tire rating.
Another desired change in conventional tires is the need for a tire to be capable of operating when in reduced pressure conditions, i.e. a run-flat tire. The majority of run-flat tires in the market place are self-supporting run-flat tires. Such tires are provided with increased thickness sidewalls that support the tire during reduced pressure conditions. The increased sidewall thickness, achieved by the use of additional rubber layers in the sidewall, can reduce a tires speed rating because of the greater weight and internal heat generated by the tire. Thus, the need to have a run-flat tire also affects the normal operation of the tire by reducing the tire's capability.